1. Field of the Invention
This invention relates to an idle control system and an idle control method for an internal combustion engine in which cylinders are selectively operated by stopping inlet and exhaust valves of some cylinders, and more particularly to an idle control system for an internal combustion engine which includes an inlet-exhaust valve stopping mechanism so as to control an idling speed to a target value. This invention also relates to an idle control method for an engine such as a modulated displacement type engine.
2. Description of the Related Art
There is known an internal combustion engine which includes a valve stopping mechanism for temporarily and selectively interrupting supply of air and fuel to some cylinders so as to reduce an engine output or fuel cost. According to various operating data, a control unit causes the valve stopping mechanism to stop the operation of some of inlet and exhaust valves of cylinders and fuel-supply thereto once the engine is operating under a predetermined condition. When the engine deviates from the predetermined condition, the inlet and exhaust valves of the stopped cylinders become operative. Then, these cylinders receive fuel and resume their normal operation with their inlet and exhaust valves opened and closed.
Conventionally, an idle speed of the engine is controlled by adjusting the amount of air to be supplied to the engine or an air-to-fuel ratio, so that the idle speed does not change abruptly when load varies. A variety of methods have been proposed to prevent change of the idle speed.
Even when the amount of intake air or fuel is increased after detecting a reduction or variation in the engine speed, such intake air or fuel does not produce an effective torque until a certain period of time lapses. The engine response depends upon characteristics of various control devices.
In an engine in which two cylinders out of four cylinders can be stopped (this state is called "partial operation of the engine" hereinafter), an inlet stroke is halved (each time a crank shaft rotates 360 degrees), and the amount of inlet air is also halved. Table 1 shows the differences of response of various control units during the full operation of the engine (i.e. when all of the four cylinders are working), and during the partial operation of the engine. In Table 1, a 1.6-liter engine (3.6-liter in an inlet system) is in the full operation at an idle speed of 700 rpm, or is in the partial operation at an idle speed of 850 rpm.
TABLE 1 ______________________________________ Control units Response for 4 cylinders 2 cylinders ______________________________________ Alternator Time constant Time constant 100 ms 100 ms Ignition After 1 stroke After 2 strokes (43 ms) (70 ms) Fuel (A/F) After 3.5 strokes 4.5 strokes (150 ms) (220 ms) Amount of air Time constant Time constant (bi-path ISC) 450 ms 700 ms ______________________________________
As can be seen from Table 1, during the partial operation of the engine, an interval of the output stroke is doubled, an equivalent capacity of the inlet system is doubled, and response of the air amount control unit is nearly halved. Furthermore, time constants for ignition timing control and fuel amount control are also increased while electric load control remains equal.
A control unit for an idle speed is designed to control various control parameters to be optimum during the full operation of the engine. Therefore, during its partial operation, the engine usually tends to suffer from reduced response and controllability, and an inconvenience such as overshooting. During the partial operation of the engine, the number of idle vibrations is half of the number of idle vibrations of the fully operating engine. The idle speed tends to be within a resonant range. There is a strong demand for preventing such a phenomenon.